In a building site and the like, a portable pneumatic tool to drive a nail and a screw into a wooden material by a pressure of compressed air has been widely used. In general, an air compressor for driving the pneumatic tool and the like is configured to convert a rotational motion of a rotational shaft of a driving part such as a motor into a reciprocal motion of a piston in a cylinder through a crankshaft of a compression part and to compress air sucked from a suction valve of the cylinder by the reciprocal motion of the piston. The compressed air compressed in the cylinder is discharged from an exhaust valve of the cylinder to an air tank through a pipe and is stored in the air tank. When compressing the gas to a high pressure, a multistage reciprocating compressor of increasing a pressure in a stepwise manner has been generally used. The high-pressure compressed air stored in the air tank is adjusted to an appropriate pressure by a decompression valve attached to the air tank and is then supplied to the pneumatic tool and the like through an air hose. The air compressor is disclosed in JP-A-2013-40586.